Like the others have said, shop second hand! I got half of my hurrican vases on fb yardsale and saved so much money.
I'm a less is more kind of gal, so I would focus on one or two rustic elements. Maybe wood disks and Mason jars with tea lights in them, or burlap runners with simple pillar candles. For diys I've seen some cute old windows as seating charts, some even have chicken wire in the panes and the names are clipped to the wire. You can paint your table numbers on wooden coins or mismatched jars.
I haven't yet decided about a florist, I love the look of dried flowers and using some of the natural wild flowers/prairie grass that's around Alberta. But that's a great tip about going for more greenery - green is actually one of my colours so it should be perfect.
The lumber yard and food as decor suggestion is excellent - hadn't even thought of that. A touch of squashes and pumpkins could really be a nice touch, thanks!
Check out Marketplace in your area on Facebook and even join wedding sell/swap in your area on Facebook as well. My fiance and I have gotten a few things from other people that have been selling their stuff from their weddings. Usually for a discounted price then in store!
I would check out marketplace and see if there are any groups where you can buy from couples who already got married. Not only is the stuff usually only used once you can get it sometimes all ready without having to do the work yourselves plus the cost savings are incredible. Also a good place to resell whatever you buy...
Are you doing all DIY or will things like flowers be done by a florist? If you're using a florist, more greenery over flowers itself will save you some money and vibe well with your rustic look.
You may be able to get tree stumps by going to a lumber shop. They're pieces that can't be used for any type of construction but look great as centerpieces. Some may be able to polish them to make food grade food boards with it.
Use food as part of your decor since your wedding is held during harvest season and many crops will be in season and low price. Think Thanksgiving harvest table setups. That way you get a pretty table and can feed the crowd.